Surprising space crystals

Monday, 7 February 2000 M. Sleath - The Lab

The surprise discovery of silicate materials in a crystallised state in space has the potential to open up an entirely new field in astronomy.

The crystals were revealed by the European Space Agency's (ESA) Infrared Space Obsevatory (ISO) and the findings reported last Friday at ESA's Villafranca station in Madrid. Silicate minerals were known to be a main component of dust in space, but detecting them in a crystallised state has been a surprise.

Silicate crystals, the most abundant minerals on Earth, are also found in great quantities around old stars and in protoplanetary discs - the discs where planets form. The new discovery allows the identification of precise silicates in astronomical objects.

Author of the report, Dutch astronomer Rens Waters of Amsterdam University, calls it a "crystalline revolution" predicting that it will open a totally new field in astronomy: astro-mineralogy.

"It's really fantastic, this possibility of identifying the silicates. Before ISO everybody thought that all silicates in space were amorphous, without a well-ordered internal structure; that means you cannot differentiate among the many different silicates existing. Now we can try to identify them and track their presence in different regions ", said Waters.

Crystals give key clues about the physical conditions and evolutionary history of crystal-bearing objects. The precise mechanisms for crystal-making are now being researched. For instance, crystals can be made by heating the material to temperatures above 1 300 degrees Centigrade and then cooling it down slowly. Those found so far by ISO are at -170 degrees Centigrade, both in stellar envelopes and in protoplanetary discs.

In the case of the old stars - red giant stars, where crystals are found to account for as much as 20% of all the surrounding dust, astronomers think that that the high temperatures near the star triggered the crystallisation of the silicates. In the protoplanetary discs some experts postulate that electric shocks - like lightning flashes - heated the dust, which cooled afterwards.

"The crystals detected by ISO in these discs have a size of about a thousandth of a millimetre. They collide with each other, forming bigger and bigger bodies. Models predict that in about ten to one hundred million years they will make planets", Waters says. "In fact, crystalline silicates are very common in our own Solar System. You also have them in the comet Hale Bopp!".

The reason why crystalline silicates had not been detected before in stars has to do with their low temperatures. The European Space Agency's infrared space observatory, ISO, operated from 1995 to May 1998. An unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe. Cold material emits mostly infrared light, which means an infrared space telescope like ESA's ISO was needed. The two high-resolution spectrometers on-board the satellite, able to detect the 'chemical fingerprint' of the crystals, did the rest.

Astronomers are sure about the discovery because those chemical fingerprints, the spectra, can be compared in laboratories with spectra from crystalline silicates found on Earth. This method has demonstrated the crystalline structure and has even already allowed the identification of some of the crystals, such as forsterite and enstatite. However, crystalline silicates are a large family and their chemical signatures can be very similar; to enlarge the list of precise crystals more work will be needed, say experts in space chemistry.

That is just one of the open questions requiring lab work. There's at least another one: crystalline silicates are found around old stars, in protoplanetary disks and in our own Solar System, but not in the space among the stars; astronomers can't explain it yet.